If is often desirable, in the manufacture of solar cells and other semiconductor devices, to plate nickel directly onto silicon so as to form electrodes or contacts for coupling the semiconductor device into an electric circuit. One of the most well known and effective ways of achieving such plating is through electroless nickel plating.
With such a process, the first step is usually to clean the outer surfaces of the silicon substrate where the nickel is to be deposited so as to remove any particles of dirt or oxide which may be present. This cleaning is desirable since the presence of such substances on the surface of the silicon tends to interfere with proper plating of the nickel onto the silicon. Cleaning may be accomplished in various ways well known to those skilled in the art, e.g. by successively immersing the substrate in suitable baths of hot organic solvents and hot chromic-sulfuric acid, followed with a bath of hydrofluoric acid and rinsing with deionized water. Once this has been done, the surface of the silicon substrate which is to receive the nickel plating is then pretreated with a catalyst. This is necessary since the silicon surface will not itself support the electroless plating process and nickel plated on an untreated silicon surface tends to adhere poorly thereon. Normally palladium is used as the catalyst, although other activators are well known to persons skilled in the art. U.S. Pat. No. 3,489,603 shows one such alternative to the palladium catalyst.
Once the surface of the silicon which is to receive the nickel has been cleaned and pretreated, the silicon is ready for electroless nickel plating. Plating is accomplished by immersing the silicon substrate in a suitable acidic or alkaline bath under appropriate conditions. A typical acidic bath might comprise nickel chloride (at 30 g/l), sodium hypophosphite (at 10 g/l), a third salt of sodium nitrite (at 10 g/l), a pH of between 4 and 6 and a temperature of approximately 190.degree. F. In such a bath the nickel chloride provides the nickel ion which is to be reduced, the hypophosphite provides the reducing agent and the third salt acts as a buffer and complexing agent for the nickel. Alternatively, an alkaline bath may be substituted. Such electroless nickel plating is well known in the art and is described in detail in such publications as Electroplating and Related Processes by J. B. Mohler (Chemical Publishing Co., Inc.; New York, 1969), Surface Preparation and Finishes for Metals edited by James A. Murphy (McGraw-Hill Book Co.; New York, 1971) and Handbook of Thin Film Technology edited by Leon I. Maissel and Reinhard Glang (McGraw-Hill Book Co.; New York, 1970).
Electroless nickel plating of this type suffers from a number of difficulties. First, the need for catalytic pretreatment of the silicon adds an additional step to the plating process. In addition, where the catalyst used is palladium the nickel tends to be plated down in an uneven pattern since the palladium has a tendency to work unevenly over the surface of the silicon. The high cost of palladium is also a negative feature, since significant amounts of palladium may be lost during the pretreatment process.
Furthermore, the chemistry of the electroless plating bath described above tends to result in the formation of some nickel phosphide which may be deposited on the silicon along with the nickel. The inclusion of this nickel phosphide in the plated nickel tends to alter the properties of the deposited nickel and may be quite undesirable depending on the applications contemplated. The phosphorous content in the deposited nickel may be held down by using an alkaline rather than an acidic bath, but the use of an alkaline bath may raise new difficulties. In particular, alkaline baths tend to etch away any aluminum exposed to the bath, thereby complicating plating where the silicon has an aluminum layer thereon. In addition, the use of an alkaline bath tends to promote the formation of an oxide layer on the surface of the silicon so as to impede the plating of nickel directly onto the silicon.
The presence of an oxide layer provides a further complication where it is desired that the deposited nickel serve as an ohmic contact. In such case the nickel layer must be sintered so as to diffuse into the silicon substrate and form a nickel silicide at the nickel/silicon interface. When an intervening oxide layer is present, the sintering must be carried out at a relatively high temperature (above 350.degree. C.) and/or for a relatively long time (40 minutes or more) in order to cause the nickel to penetrate the oxide layer and diffuse into the silicon and form an ohmic contact. However, where the substrate is a shallow junction device such as a solar cell and the nickel layer is on the surface of the substrate nearest the junction, there is a tendency for the nickel to diffuse deep enough to shunt or short out the device, especially where the oxide layer is very thin or non-existent. In this context a shallow junction silicon device is one where the junction is about 1.0 micron or less below the surface on which the nickel is deposited.
As a result, one of the objects of the present invention is to develop a method of plating nickel directly on silicon which does not use the electroless nickel plating process just described. Another object is to provide a plating process which renders unnecessary any catalyzing pretreatment of the silicon surface prior to plating with nickel. Yet another object is to provide a method of achieving strong nickel adherence to the silicon. Still another object is to provide a method of plating nickel directly onto silicon without producing nickel phosphide contaminants. And another object is to provide a plating method utilizing a bath which does not etch away aluminum exposed to the bath. Yet another object is to provide a plating method which does not require the use of expensive palladium. Still another object is to provide a method of making nickel ohmic contacts on silicon devices where the sintering may be carried out at relatively low temperatures (350.degree. C. or less) and within relatively short times (less than 40 minutes). A further object is to provide a simple and reliable method of making nickel ohmic contacts where the reproducibility of high quality contacts is goods or better.